"It's been difficult," Monk added. "He came back into training, but he is still feeling soreness. We had to pull him out of training and he has to have an injection.

"It's nothing serious, but he needs two days' complete rest and we can build him up. He won't be available (against Napoli), which is disappointing for him and us. He was eager to get out there, but sometimes you can push yourself too much."